Ejector assisted cycles are known to offer significant COP and capacity improvements, especially for transcritical CO2 systems, when operated at the ejector design conditions. However, significant deviation from design conditions can lead to poor ejector and cycle performance. In this paper, a numerical model of a transcritical CO2 ejector cycle accounting for off-design ejector performance is developed. Experimental data from a CO2 ejector cycle, taken over a range of compressor speeds and ambient temperatures, is used to develop empirical models for the ejector and compressor. The evaporator in the system is a counterflow, microchannel heat exchanger and is simulated using a finite difference model capable of accounting for the heat transfer and pressure drop throughout the heat exchanger. The models of the individual components and cycle are validated using experimental data; good agreement between model and experiment is obtained.